The present invention generally relates to cementitious compositions. The invention particularly relates to cementitious compositions containing internal curing agents and mineral admixtures.
Cementitious compositions for forming high-performance concrete (HPC) and ultra-high-performance concrete (UHPC) commonly include various additives intended to improve various properties of the compositions and the concrete they produce. Such additives may include any ingredients other than Portland cement, water, and aggregate that are added to the composition before or during mixing. These additives are referred to herein as admixtures.
Portland-pozzolan blended cement typically contains 20-25 wt. % of a pozzolan (also referred to herein as a pozzolanic material), resulting in significant energy and cost savings as less Portland cement can be used. A pozzolan is defined by ASTM C125 (ASTM: American Society for Testing Materials) as a siliceous or siliceous/aluminous material that will chemically react with calcium hydroxide (Ca(OH)2) and/or calcium cations (Ca2+) in the presence of water (referred to herein as pozzolanic reaction or reactions) at ordinary temperatures to form compounds with cementitious properties (including C—S—H which provides long-term strength and durability to concrete). During setting, pozzolanic reactions result in improved resistance to thermal cracking due to a relatively low heat of hydration (roughly half that of pure Portland cement). Additionally, the pozzolanic reaction products of very fine (sub-micron) siliceous materials are more efficient at filling capillary spaces within the cement paste without the formation of large expansive pressures and phases within in the concrete (i.e., typical of alkali-silica reactions (ASR), which can lead to cracking and decreased concrete durability), resulting in a refined pore size distribution (i.e., replacing macropores with microporous material) thus improving the strength, durability, and impermeability of the hardened concrete.
Common pozzolans include silica fume and rice husk ash which are both industrial waste products that consist of pure amorphous silica in the form of particles. Pozzolanic particles and cementitious mixtures are well described in U.S. Pat. No. 7,442,248 issued to Timmons on Oct. 28, 2008. The contents of this patent are incorporated herein by reference in their entirety. Pozzolanic particles employed in cementitious applications are typically in the size range of 50-150 nanometers. Pozzolanic particles smaller in size, for example in the range 10-50 nanometers, have higher surface area and hence increased reactivity. Smaller particles are also more likely to react completely and form C—S—H without forming expansive phases typical of ASR. However, particles in this smaller size range are difficult to handle due to the hazardous nature of these particles if inhaled. Additionally, these smaller particles typically decrease the workability of the cementitious mixture, an undesirable result which in turn requires greater pumping pressures and increases the difficulty of mixture placement.
In an effort to lower their water-to-binder ratios, cementitious mixtures used to produce HPCs and UHPCs may include internal curing agents. These agents, typically wet porous aggregate or swollen superabsorbent polymer (SAP) hydrogels, provide a continuous supply of water during curing, thus counteracting self-desiccation and reducing or potentially eliminating autogenous shrinkage and cracking of the cement and achieving a corresponding increase in compressive strength and durability.
SAP hydrogel particles may swell to as much as 100 times their original weight in the presence of water as shown in image (a) of FIG. 1. An exemplary SAP hydrogel may be composed of poly(acrylamide(AM)-acrylic acid(AA)) polymer molecules that are chemically cross-linked to neighboring molecules to form a three-dimensional copolymer network that will absorb fluid and swell to form a hydrogel under alkaline conditions. The SAP hydrogels are produced and added to cement as dry particles, and therefore are relatively easy to transport and incorporate into cementitious mixtures. In addition to their strong hydration performance, SAP hydrogels may reduce thermal expansion and tensile creep and enhance freeze/thaw resistance of the cementitious mixture. However, upon water release during cement curing, the particles leave behind voids in the hardened cement, and therefore in the resulting concrete as shown in FIG. 4.
In view of the above, there is an ongoing desire for admixtures capable of modifying or improving the properties and characteristics of cementitious mixtures and concrete formed therewith.